Next week I am flying to Gatwick airport in London, and then I must go to Abbey Wood train station (in Greenwich) which is where I am staying. I believe the easiest route is to use trains, connecting at London Bridge, but I am a bit confused with the fares. What would be the cheapest fare for this route? I am not planning to use an Oyster card for my stay as for the next 7 days I will use a Zones 1-4 Travel Card, but I also wouldn't like to burn one of my travel card days for this journey (I will arrive late at night, so I don't see a point of getting one day less on my travel card for a single journey). Is there an option to buy a ticket for this route only (so no 1 day ticket) without having an Oyster card? Or what would be the cheapest option for me?
The best solution pricewise would be buying an Oyster card or getting a contactless bank card if those are cheaper to get at your bank.
You may need to buy the Oyster card before arriving in London, you certainly will need to if you want to use a Visitor Oyster card.
With an Oyster card your journey will cost you £8, (off peak,) a one way train ticket (also off peak) is £17.40.
The cost of a Visitor Oyster Card is £3 more than the credit on it, as you can see on this site. So even for a single journey you are better off using that card.
You can get any remaining credit paid out when you use a ticket machine at the end of your stay, if the stay is more than 3 days. The card will remain yours but without value. If you plan to return in the near future, or have friends who plan a London visit, you can also leave the credit on and use the card at a later date.
Credit remains valid, no end date.
If you have a contact less bank card, the same prices for travel apply, without the need to have an extra card.
All prices taken from the comments on the question, except where I put in a link.